1. Field of the Invention
This invention relates to switch-mode power converters and more specifically to an improved magnetic core structure that reduces the fringing flux and winding eddy current losses by eliminating the air gap.
2. Description of the Related Art
Switch-mode power converters are key components in many military and commercial systems for the conversion, control and conditioning of electrical power and they often govern size and performance. Power density, efficiency and reliability are key metrics used to evaluate power converters. Transformers and inductors used within these power converters constitute a significant percentage of their volume and weight, hence determine their power density, specific power, efficiency and reliability.
Gapping of magnetic cores is standard practice for inductor assemblies to provide localized energy storage and prevent core saturation. The air gap can withstand very high magnetic fields, hence supports the applied magnetomotive force almost entirely and provides local energy storage. Due to its low permeability compared to the core material, the air gap increases the overall magnetic reluctance of the core thereby maintaining the flux and the flux density below the saturation limits of the core material. The high permeability core material provides a path for the closure of the magnetic flux lines and also houses the winding turns to generate the required magnetomotive force in the core.
Integrated magnetics provides a technique to combine multiple inductors and/or transformers in a single magnetic core. It is amenable to interleaved current multiplier topologies where the input or output current is shared between multiple inductors. Integrated magnetics offers several advantages such as improved power density and reduced cost due to elimination of discrete magnetic components, reduced switching ripple in inductor currents over a discrete implementation and higher efficiency due to reduced magnetic core and copper losses. Planar magnetics, where transformer and inductor windings are synthesized as copper traces on a multi-layer printed circuit board (PCB) offer several advantages, especially for low-power dc—dc converter applications, such as low converter profile, improved power density and reliability, reduced cost, and close coupling between the windings.
The integrated magnetics assembly 10 shown in FIG. 1 for a current-doubler rectifier (CDR) comprises an E-core 12 and plate 14 wound with split-primary windings 16 and 18, secondary windings 20 and 22, and an inductor winding 24 (See U.S. Pat. No. 6,549,436). This assembly integrates a transformer and three inductors in a single E-core. As a result, the magnetic flux in the core consists of transformer and inductive components. The center leg of the E-core is in the inductive flux path, hence is gapped to prevent core saturation and provide energy storage. A high permeability path is maintained for a transformer flux component to ensure good magnetic coupling between the primary and secondary windings. The inductive flux components flow through the outer legs 26, 28, and the center leg 30, the low permeability air gap 32, and complete through the top plate 14 and the base 30. The transformer component of the flux circulates in the outer legs 26 and 28, the top plate 14 and the base 30, which form a high permeability path around the E-core 12. The center-leg winding is used to increase the effective filtering inductance and carries the full load current continuously.
As shown in FIGS. 2a and 2b, the integrated magnetics assembly 10 is implemented using planar windings synthesized with a multi-layer PCB 33 having copper traces that form horizontal windings in the plane of the PCB. E-core 12 is positioned underneath the PCB so that its outer legs 26 and 28 extend through holes in the PCB that coincide with the centers of primary and secondary windings 16 and 20 and 18 and 22, respectively, and its center leg 30 extends through a hole that coincides with inductor winding 24. Plate 14 rests on the outer legs forming the requisite air gap 32 with the center leg.
Inductance is primarily determined by the core reluctance and the number of turns. Since the relative permeability of air is negligible compared to that of the core material, the reluctance, along the inductive flux path, of an E-core with a gapped center leg is dominated by that of the air gap. One limitation on the cross sectional area of the center leg and hence of the air gap is fringing flux. Like bright light from one room leaking under a door into a dark second room, a portion of the flux from the air gap 32 spills onto the width of the core window 36 and impinges on the planar windings therein. This is schematically illustrated in FIG. 3. The fringing flux lines 34 are normal to the plane of the windings on the PCB 33, as shown in FIG. 4, resulting in the induction of large eddy currents 38 in the windings. Fringing flux affects converter metrics in two ways. (i) It induces eddy currents in the planar windings, which result in I2R losses and poor efficiency (ii) Reduced inductance due to loss of flux from the main magnetic path. One way to reduce the eddy currents is to place the planar windings a safe distance away from the air gap. To do this, the outer legs may be far from the center leg, thereby making the window wider, or the outer legs may be made taller, thereby increasing the height of core window 36 so that the windings may be positioned closer to the base and far enough away from the air gap 32. These two solutions result in either a wider E-core or a taller E-core, both of which result in reduced power density and poor utilization of the core volume. If the number of planar PCB layers increases to accommodate more turns for higher inductance, it may become inevitable that some of the winding layers be close enough to the air gap 32 that they will suffer from high eddy current losses due to the strong fringing flux.
Loss of inductance due to fringing flux results in increased switching ripple and hence higher I2R losses in the windings and the semiconductor devices. In addition, a higher output capacitance is required to accommodate the higher inductor current ripple resulting in reduced power density.